Max Zs tables for Type B, C and D circuit breakers
Maximum earth fault loop impedance for BS EN 60898 circuit breakers and BS EN 61009 RCBOs, for 0.4 s disconnection at Uo 230 V — plus the 80% figure to compare with a reading taken at room temperature.
How to use these tables
- Find the device type and rating.
- Compare your measured Zs with the 80% column — the full table value assumes conductors at their operating temperature.
- For 5 s disconnection (distribution circuits and larger final circuits) and for fuses, use the other tables in BS 7671 Chapter 41.
- Always check the current edition and amendment of BS 7671 for the values in force.
Type B circuit breakers (0.4 s)
| Rating | B6 | B10 | B16 | B20 | B25 | B32 | B40 | B50 |
|---|---|---|---|---|---|---|---|---|
| Max Zs (Ω) | 7.28 | 4.37 | 2.73 | 2.19 | 1.75 | 1.37 | 1.09 | 0.87 |
| 80% (Ω) | 5.82 | 3.50 | 2.18 | 1.75 | 1.40 | 1.10 | 0.87 | 0.70 |
Type C circuit breakers (0.4 s)
| Rating | C6 | C10 | C16 | C20 | C25 | C32 | C40 | C50 |
|---|---|---|---|---|---|---|---|---|
| Max Zs (Ω) | 3.64 | 2.19 | 1.37 | 1.09 | 0.87 | 0.68 | 0.55 | 0.44 |
| 80% (Ω) | 2.91 | 1.75 | 1.10 | 0.87 | 0.70 | 0.54 | 0.44 | 0.35 |
Type D circuit breakers (0.4 s)
| Rating | D6 | D10 | D16 | D20 | D25 | D32 | D40 | D50 |
|---|---|---|---|---|---|---|---|---|
| Max Zs (Ω) | 1.82 | 1.09 | 0.68 | 0.55 | 0.44 | 0.34 | 0.27 | 0.22 |
| 80% (Ω) | 1.46 | 0.87 | 0.54 | 0.44 | 0.35 | 0.27 | 0.22 | 0.18 |
iZapp’s 3D Testing Bench lets you do this test on a virtual board with real-looking kit: place the probes, read the tester and record the result on a practice schedule. Some attempts have a fault planted for you to find.
Frequently asked questions
What is the max Zs for a B32?
1.37 Ω for 0.4 s disconnection (BS 7671 Table 41.3). Measured at room temperature, compare with about 1.10 Ω (80%).
What is the max Zs for a C32?
0.68 Ω (Table 41.3). The 80% value to compare a site reading with is about 0.54 Ω.
Why use 80%?
Conductor resistance rises with temperature under fault and load. Testing cold, 80% of the tabulated value allows for that rise (IET Guidance Note 3 rule of thumb).
Last updated 2026-10-14. Training summary of BS 7671 and IET Guidance Note 3 practice, written by NAPIT-registered electricians. Always check the current edition and amendment of BS 7671 and your scheme’s requirements.